In communication devices designed to transmit signals, such as cellular devices, power amplifier circuits are typically used to amplify a desired signal to allow proper transmission. Power amplifier circuits may, for example, be implemented in Complimentary Metal Oxide Semiconductor (CMOS) technology or Bipolar Junction Transistor (BJT) technology. The power amplifier circuits may, for example, comprise of two or more cascaded gain stages, a driver stage and a power stage. The power stage may include CMOS or BJT transistors. Both CMOS and BJT transistors have breakdown voltages, which if exceeded may result in damage to the transistor. Breakdown voltage is a maximum voltage, which when be applied across any two terminals of a transistor may cause transistor damage. For example, there is a maximum voltage, which can be applied across the drain and source terminal of the CMOS transistor, or the collector and emitter terminal of the BJT transistor. When the breakdown voltage is exceeded an avalanche current may be created in the transistor, which results in a large current flowing through the transistor and thus a significant increase in heat that may damage the transistor and degrade transistor performance. Transistor breakdown can also occur if the maximum safe operating voltage across any other pair of transistor terminals, such as the gate to drain voltage or gate to source voltage. If the gate to drain voltage or gate to source voltage exceeds the respective breakdown voltage, gate oxide breakdown way occur. Oxide breakdown is when conduction path is created from the anode to the cathode through the gate oxide layer of the transistor. When the transistors utilized in the power amplifier circuit are damaged, the transistors may operate in an unpredictable manner or cease to operate at all. Thus, power amplifier circuit reliability is severely compromised by transistor breakdown voltage occurrence in the power stage. Therefore, when designing transistor circuits it is of paramount importance to design the circuit such that the voltage across the terminals of the transistor does not exceed the transistor's breakdown voltage.
However, to maximize power amplifier circuit efficiency the output of the power amplifier circuit needs to be capable of creating an output voltage swing from the negative voltage supply value up to the positive voltage supply value. In many embodiments, for example, the output voltage swing may be greater than twice the battery voltage. For example, if the device battery has a voltage level of 4.2 volts, the power amplifier circuit would need to be capable of creating an output voltage swing of 8.4 volts or more. However, a typical transistor, for example, may only be able to withstand up to 3 volts across its terminals before it reaches the breakdown region. To alleviate the voltage applied across the terminals of the power amplifier circuit transistors, multiple transistors may be coupled in a cascode configuration. The cascode configuration, for example, may comprise three transistors coupled in a cascode configuration thus reducing the voltage applied across each transistor. While this may prevent each transistor from reaching the breakdown voltage, the drain to gate voltage of the top transistor may cause damage when the output voltage is peaked. Further, if the output of the power amplifier circuit is attached to an antenna, a variation to the Voltage Standing Wave Ratio (VSWR) of the antenna may cause the output voltage of the power amplifier circuit increase to a even greater voltage level. Lastly, the voltage provided by the device battery may vary, for example, from 3.2 volts to 4.2 volts.
Therefore, there is a need for a power amplifier circuit design, which provides full output voltage swing capability and thus maximum efficiency while protecting the power amplifier circuit transistors from reaching the breakdown voltage.